1. Field of the Invention
This invention relates to chemical mechanical polishing in semiconductor fabrication, and more particularly, to cleaning of a fixed abrasive polishing pad.
2. Description of Prior Art
Current processing of semiconductor wafers includes deposition of several layers containing a plurality of patterns and vertically interconnected elements for forming an integrated circuit (IC) device. A typical integrated circuit may have six or more layers of metal wiring that are isolated by interlayer dielectric layers. As the number of layers increases, surface irregularities of the wafer will be a problem for acceptable yield and long-term reliability of a chip. Therefore, a planarization process should be performed on a processed wafer to obtain a smooth surface. In the 1980's, IBM had introduced techniques for chemical mechanical polishing (CMP) of processed wafers. CMP was firstly applied to planarization for inter-metal dielectric (IMD) in backend process. CMP can effectively ensure surface planarization of a wafer or a processed substrate. A very thin layer of material can be removed from a surface of a work piece by a combined chemical and mechanical interaction. CMP can be used for ultra precision surface machining CMP has become a fast-growing and valuable process for IC fabrication in recent years. The basic principle of CMP is to apply a mechanical force towards a surface of the wafer, thereby generate a motive power leading to cracking corrosion in a surface thin layer with which chemical substance of abrasive liquid reacts to improve etching rate.
With slurries as polishing solutions, particles removed from the wafer or substrate to be polished are together rinsed with the slurries. A large amount of slurries will be used. An alternative process is the use of a fixed abrasive polishing pad, where an abrasive is contained within the pad.
The principle of a fixed abrasives chemical mechanical polishing is two-body abrasion, that is, a work piece is polished by abrasives fixed on a polishing pad, with an inevitable consequence, that some byproducts of micro particles during polishing such as some dropped fixed abrasive particles in operation, if not cleared away, not only affect the polishing rate, but also scratch the work piece, then affect the polishing precision and yield. To solve this problem, a prior art technique provides a chemical mechanical polishing table with a caterpillar regulating brush that at least includes a main body structure having a long shaft and a caterpillar on which multiple hard particles and rollers are distributed. The caterpillar covers the external surface of the main body structure and revolves at a fixed speed. The rollers are arranged axially and in parallel to an internal side of caterpillar and are in contact with the caterpillar. The rollers rotate under the drive of the caterpillar. There are multiple particles on a surface of the caterpillar for scraping a surface of a polishing pad to remove the residual impurities remained on the polishing pad. The caterpillar regulating brush also includes a cleaning device for cleaning the caterpillar to remove residual impurities during the caterpillar regulating brush scraping. Although a part of the polishing byproducts may be removed through this technique, this technique has low efficiency and is limited to cleaning capability for low particles precision on the caterpillar and is easily polluted by the byproducts, so the technique may not be suitable to be used as the cleaning device. American Patent Application No US2002/0090896A1 discloses a pad cleaning technique for a CMP system that uses for a fixed abrasive chemical mechanical polishing. The pad cleaning for a CMP system use one or more nozzles that are arranged at an angle relative to a surface of a cleaned polishing pad for spraying liquids at pressures of about 30 psi to about 300 psi or greater to clear away byproducts of micro particles. Although this disclosure improves cleaning efficiency, the micro particles remained on the surface of the polishing pad cannot be removed in time by simply cleaning using pressure, and scratching of a work piece cannot be avoided as the byproducts may migrate to the polishing interface.
A conventional chemical mechanical polishing equipment includes a means for holding a wafer or substrate to be polished (referred to as a “wafer chuck”), a polishing pad, a means to support the polishing pad (referred to as a “platen”), a supply mechanism for supplying abrasive slurry. The wafer is held by the wafer chuck having the side to be polished facing down towards the polishing pad. The polishing pad is not abrasive. During the chemical mechanical polishing, the rotating wafer chuck presses the wafer with an appropriate pressure towards the polishing pad. The slurry is delivered to the surface of the polishing pad. The slurry contains submicron abrasives or nano-abrasives and chemical solution that fill a space between the wafer and the polishing pad, and produce chemical reactions on the surface of the wafer. Reactants forming on the surface of the wafer are removed under a mechanical action of the abrasives. The polishing pad generally includes polymer, such as polyurethane, polyester, and the like.
FIG. 1 schematically illustrates a conventional system for cleaning a fixed abrasive polishing pad. The conventional cleaning system includes a nozzle for delivering a polishing liquid which includes deionized water. A wafer to be polished 1 is fixed on the wafer chuck by means of vacuum. During the polishing operation of wafer 1, a platen (e.g., polishing table) rotates a fixed abrasive polishing pad 2 with a rotation speed between 50 revolutions per minutes (rpm) and 200 rpm. In a polishing contacting region, a prescribed amount of pressure and a rotation speed difference between the polishing pad 2 and the wafer 1 are provided for polishing wafer 1. A cleaning device 3 located outside the polishing contacting region is coupled to a supply system of cleaning liquid (e.g., deionized water). While polishing wafer 1, the cleaning device 3 supplies a cleaning liquid (deionized water) through an inlet 4 and the cleaning water is ejected through a nozzle to remove micro particles that are deposited in the grooves of the fixed abrasive polishing pad 2. The cleaning liquids that carry micro byproducts are then discharged, and the polishing pad is cleaned.
In the fixed abrasive chemical mechanical polishing system described above, an amount of hydraulic pressure ranging from 30 psi to 300 psi is needed to wash away the micro particles disposed in the grooves of the polishing pad. However, a homogeneous hydraulic pressure of the cleaning liquid is difficult to be maintained throughout the polishing pad. For example, the washing pressure in the region of the polishing pad which is more distant from the nozzle is weak relatively to the washing pressure in the region close to the nozzle; the conventional system may not ensure that micro byproducts can be effectively removed.
Therefore, there is a need to have a method and system for cleaning a fixed abrasive polishing pad that can effectively remove micro particles that may cause scratches and defects in the surface of the wafer during polishing.